1. Field of the Invention
The invention in general relates to the monitoring of rotary machinery, and particularly to the monitoring of blading arrangements on turbines.
2. Description of the Prior Art
In order to reduce the unscheduled outages in turbine generator systems, and to prolong the life of the equipment, use is made of monitoring apparatus whereby abnormal operating conditions may be detected and remedied.
In such monitoring arrangements, it would be useful to know the distance between the turbine blading and its adjacent housing under operating conditions. Such information would not only provide an indication as to possible housing distortion but would also give clues to other possible sources of trouble such as turbine blade vibration, critical speed shaft vibration and a possibility of crack situations, by way of example.
In a multistage turbine, a selected blade row, generally the last, may be chosen for monitoring purposes and for obtaining blade-to-housing distance a plurality of housing-mounted proximity sensors may be provided. One type of sensor in common use is of the eddy-current type having a certain impedance which changes in response to the sensor's proximity to a conducting member. The sensor includes an AC excited coil, the inductance and resistance of which change as the coil is brought in close proximity to a metallic object. Since the coil's inductance and resistance would also change in response to temperature-induced electrical conductivity and magnetic permeability changes, many sensors include dual coils utilizing a particular bridge arrangement to nullify the effect of temperature-induced coil impedance changes so that the sensor output signal is free of any signal component which is non-proximity related.
A problem arises, however, in that in the operating environment of a turbine, tremendous variations in temperature occur, the effects of which may be difficult or inconvenient to null due to instrument inaccessability or the requirement for virtually constant recalibration, for example. Since the sensor output signals are critical in the monitoring of a multi-million dollar machine, it is imperative that the signals be as highly accurate as possible.
The present invention fills the need for a highly accurate turbine blading monitoring system.